User manual

At small displayed currents the burden voltage is usually not an issue, but at larger displayed currents
(relative to Full-Scale) the burden voltage can be very high, even in the order of several volts! This
can often force you to use a higher current range (with a lower current shut resistor), with subsequent
loss of resolution and (often) accuracy.
You may have encountered this many times before as your circuit either not working or “playing up”
on too low a current range – that’s the burden voltage at work, starving your circuit of the voltage it
needs to function correctly. You usually have no option but to reluctantly switch to a higher current
range to lessen the effect.
The problem can also be highlighted with the many 4 ½ digit or “10000 count” meters on the market.
In theory they allow you to get an extra digit of resolution over a 3½ digit meter. But you may now find
yourself trying to measure for example 990.0µA on the 1mA range with a burden voltage of just under
1V. Can your circuit really handle a 1V drop?
The burden voltage of a multimeter is determined primarily by the shunt resistor used for
measurement, but on the higher current ranges (mA & A) it also includes the protection fuse
resistance, and to a much lesser extent, any switch and test lead contact resistance. Some
manufacturers will specify it as a total, or just the shunt resistor, or in many cases not mention it at all!
Some meters will specify it as a maximum voltage drop only. For example, “300mV max”. In this
case, to get the mV/A value you simply divide that voltage by the full-scale range current.
Low Voltage Current Measurement
The recent trend toward low voltage microcontrollers and other silicon devices (some operating as
low as 1V or less!) have really highlighted the need for consideration of the burden voltage when
measuring the chip current from its supply rail. 3.3V supplies have been entrenched everywhere for a
long time now, and the trend is going lower.
A common task these days is to measure the accurate sleep and operating current of a
microcontroller for instance. Indeed, with the lower the supply voltage of today’s battery powered
circuits, the greater the requirement to more accurately measure the supply current. So the industry
has changed, but meters haven’t really kept up with the pace when it comes to accurate current
measurement. You think they may have as meters are getter more “accurate” for less cost, but that’s
only part of the story.
Let’s look at how the supply voltage can impact your current measurement, or vice-versa as the case
may be:
Lets say you want to measure the supply current of a chip or circuit taking 200mA using say a 4000
count meter on the 400mA range. A not uncommon scenario, and one you would think would be
pretty easy for any mutlimeter to handle right? – Hold on…
A typical high end “accurate” multimeter will have a “low” 1mV/mA burden voltage (about as low as it
gets), so this means the meter will drop 200mV across its shunt resistor at 200mA. This represents
an almost tolerable 4% (200mV / 5V * 100) of a 5V supply voltage. This may not be a big deal if your
supply voltage is spot on 5V, as your chip will get 4.8V and still be within spec. But what if it’s already
say 4.8V?, your chip or circuit is now getting 4.6V and may be outside of its operational spec. This
already shows the limitation of the current range on a typically mid to high end multimeter. Not to
even mention the circuit current can differ when you lower the rail by 0.2V…